The present invention relates to a thermal fixing roller for use in an electronic copying machine, and more particularly, to a thermal fixing roller for thermally fixing a dry type developing agent consisting principally of colored toner and resin on a support in an electronic copying machine.
In a heretofore known thermal fixing roller, a heater is provided on the inside of a metallic support of cylindrical shape, and the surface of the thermal fixing roller is heated by this heater.
However, since this heating process relies upon thermal radiation from the heater, a heat-up time, that is, a time period necessitated from start of the copying machine until the copying machine becomes available is long, and it takes about 1 to 2 minutes.
Hence, a thermal fixing roller of the so-called planar heat-generating resistor type, is employed, in which a planar heat-generating resistor is provided on a surface of a support for the purpose of shortening the above-mentioned heat-up time, an electric current is passed from one end of the resistor towards the other end, and the roller surface is directly heated by Joule's heat generated at this time.
However, as the thickness of this planar heater is uniform over its entire length and the opposite end portions of the heater is liable to be cooled as compared to its central portion, surface temperature distribution in the axial direction of the thermal fixing roller is such that the temperature at the opposite end portions of the roller is lower than that at the central portion. Consequently, it becomes difficult to attain a uniform picture.
Therefore, in the prior art, a thermal fixing roller in which equalization of the above-mentioned temperature distribution was attempted by forming a film of a resistor on a thermal fixing roller in a fixed thickness, scraping this film of a resistor in the proximities of the opposite ends of the roller, and increasing resistances of these portions, was known (See Japanese Laid-Open Patent Specification No. 59-154476(1984)). However, in this example of the prior art, a troublesome work of scraping a film of a resistor in the proximities of the opposite ends of the roller is necessitated and a lot of time and labor is necessary therefor, which causes rise of cost of a roller.
In addition, since the thickness of the resistor film is thin, for example, 50 .mu.m, it is extremely difficult to scrape this film up to a desired thickness, and therefore, temperature distribution on a roller surface is liable to become uneven.